Revert "drm/virtio: Use dma_buf from GEM object instance"

This reverts commit 415cb45895f43015515473fbc40563ca5eec9a7c.

The dma_buf field in struct drm_gem_object is not stable over the
object instance's lifetime. The field becomes NULL when user space
releases the final GEM handle on the buffer object. This resulted
in a NULL-pointer deref.

Workarounds in commit 5307dce878d4 ("drm/gem: Acquire references on
GEM handles for framebuffers") and commit f6bfc9afc751 ("drm/framebuffer:
Acquire internal references on GEM handles") only solved the problem
partially. They especially don't work for buffer objects without a DRM
framebuffer associated.

Hence, this revert to going back to using .import_attach->dmabuf.

Signed-off-by: Thomas Zimmermann <[email protected]>
Reviewed-by: Simona Vetter <[email protected]>
Acked-by: Christian König <[email protected]>
Acked-by: Zack Rusin <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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